1. Field of the Invention
The present invention relates to an optical disc apparatus, such as an optical information recording and reproducing apparatus or optical information reproducing apparatus for recording information on an optical disc, such as a photo magnetic disc, and/or reproducing the recorded information. More specifically, the present invention relates to a galvanomirror unit incorporated in an optical system thereof.
2. Description of Related Art
In a known optical apparatus, an objective lens, which converges a laser beam from a laser source onto an optical disc and/or receives the light reflected from the optical disc, is linearly moved across the tracks of the optical disc in the radial direction. A known optical disc apparatus usually includes a galvanomirror unit for adjusting the laser beam emitted from the laser source, so as to make the beam parallel with the direction of movement of the objective lens within a vertical plane lying in the radial direction of the optical disc. The galvanomirror unit has a galvanomirror which rotates to vary the direction of reflection of the laser beam with respect to the objective lens.
If a positional deviation occurs between the focal point of the laser beam on the optical disc and the track of the optical disc, the reflecting surface of the galvanomirror is rotated about a horizontal axis and is deflected within the vertical plane (pitch direction) in accordance with a tracking error signal issued from a servo-sensor which detects light reflected by the optical disc, so that the optical axis of the reflected laser is lo deflected in the vertical plane due to the change in the angular direction of the reflecting surface. As a result, the focal point (convergence point) of the laser beam on the optical disc is slightly adjusted in the radial direction (cross-track direction) to absorb the positional deviation, thereby carrying out the tracking operation.
In such a known optical disc apparatus, the galvanomirror is supported by an elastic member so that the inclination angle of the reflecting surface of the galvanomirror may be adjusted about a horizontal axis. However, the elastic support tends to bend downward due to the weight of the galvanomirror causing the initial position (normal state) of the galvanomirror to vary, which may, for example, result in a deviation from the vertical plane. Consequently, it is necessary for the control mechanism to apply some external force to the galvanomirror so as to maintain the galvanomirror's angular position in some predetermined initial state. This complicates the control mechanism. Furthermore, precise control of the galvanomirror inclination angle cannot be effected when the elastic support is initially deformed because an irregular (i.e., unsteady or asymmetric) force is required for angular adjustment of the galvanomirror.